ABB REB650, Technical Manual

Page: 110 / 504

7.3.7 Operation principle
The sampled analog phase currents are pre-processed and for each phase current the
true RMS value of each phase current is derived. These phase current values are fed to
the Thermal overload protection, two time constants (TRPTTR, 49).
From the largest of the three phase currents a relative final temperature (heat content)
is calculated according to the expression:
2
final
ref
I
I
æ ö
Q = ç
÷
ç ÷
è ø
EQUATION1171 V1 EN
(Equation 17)
where:
I is the largest phase current
I
ref
is a given reference current
If this calculated relative temperature is larger than the relative temperature level
corresponding to the set operate (trip) current a pickup output signal PICKUP is activated.
The actual temperature at the actual execution cycle is calculated as:
If
final n
Q > Q
EQUATION1172 V1 EN
(Equation 18)
( )
1 1
1
t
n n final n
e
t
D
-
- -
æ ö
Q = Q + Q - Q × -
ç ÷
è ø
EQUATION1173 V1 EN
(Equation 19)
If
final n
Q < Q
EQUATION1174 V1 EN
(Equation 20)
( )
1
t
n final final n
e
t
D
-
-
Q = Q - Q - Q ×
EQUATION1175 V1 EN
(Equation 21)
where:
Q
n
is the calculated present temperature
Table continues on next page
Section 7
1MRK 505 277-UUS C
Current protection
104
Technical Manual